Polycarbonates have recently come to be widely used in numerous fields due to their superior heat resistance, impact resistance and transparency. Numerous studies have previously been conducted on methods for producing these polycarbonates. Among these, polycarbonates derived from 2,2-bis(4-hydroxyphenyl)propane (to be referred to as “bisphenol A”), for example, have been industrialized by both interfacial polymerization and melt polymerization production methods.
According to this interfacial polymerization, polycarbonate is produced from bisphenol A and phosgene, but it requires the use of toxic phosgene. In addition, this method also has problems such as corrosion of equipment by chlorine-containing compounds such by-product hydrogen chloride and sodium chloride as well as methylene chloride used in large amounts as a solvent, and difficulty in removing impurities such as sodium chloride as well as residual methylene chloride that have an effect on polymer properties.
On the other hand, melt polymerization, consisting of polymerizing, for example, bisphenol A and diphenyl carbonate in a molten state by a transesterification reaction while removing by-product aromatic monohydroxy compounds (phenol in the case of reacting bisphenol A and diphenyl carbonate) has long been known as a method for producing polycarbonates from aromatic dihydroxy compounds and diaryl carbonates.
Differing from interfacial polymerization, melt polymerization offers advantages such as not using a solvent, but it also has the intrinsic problem of polymer viscosity in the system increasing rapidly as polymerization progresses, thereby making it difficult to efficiently remove by-product aromatic monohydroxy compounds outside the system while also making it difficult to increase the degree of polymerization due to an extreme decrease in the reaction rate. Accordingly, an effective method is sought for producing high molecular weight aromatic polycarbonate resin using melt polymerization.
Various contrivances have been proposed for extracting aromatic monohydroxy compounds from highly viscous polymers for use as methods that solve the aforementioned problems (Patent Document 1: Japanese Examined Patent Publication No. S50-19600, Patent Document 2: Japanese Unexamined Patent Publication No. H2-153923, and Patent Document 3: U.S. Pat. No. 5,521,275).
However, in the methods disclosed in these publications, it is not possible to adequately increase the molecular weight of the resulting polycarbonate. When highly polymerization is conducted by a method that uses a large amount of catalyst (Patent Document 2, Patent Document 3) or under severe conditions in the manner of applying high shear (Patent Document 1) as previously described, there are considerable detrimental effects on the physical properties of the resin, such as inferior resin hue or the progression of crosslinking reactions.
In addition, methods have also been proposed for enhancing the degree of polymerization of polycarbonates by adding a polymerization promoter or linking agent and the like to the reaction system during melt polymerization (Patent Documents 4 to 10). In addition, although the objective is not necessarily the same, methods consisting of the addition of a diol compound to a reaction system between a dihydroxy compound and diester carbonate have previously been proposed (Patent Documents 11 and 12).
However, these methods also have problems such as failure to inadequately increase the degree of polymerization or causing decreases in the inherent physical properties of the resulting polycarbonate resin (such as thermal stability, impact resistance or hue).
In this manner, since conventional methods for producing high molecular weight aromatic polycarbonates have numerous problems, there is a strong desire for the development of a production method capable of retaining the inherent favorable qualities of polycarbonates while achieving an adequate highly polymerization.
The inventors of the present invention previously proposed a method for producing a high molecular weight aromatic polycarbonate resin capable of retaining the favorable qualities of aromatic polycarbonate resins while adequately highly polymerizing (Patent Document 13). This method consisted of highly polymerizing by linking an aromatic polycarbonate prepolymer having an extremely low terminal hydroxyl group concentration with a linking agent composed of an aliphatic diol compound, which has a specific structure and has an aliphatic group that bonds to a terminal hydroxyl group contributing to the formation of a carbonate bond by transesterification (to simply be referred to as an “aliphatic diol compound”), by copolymerizing in the presence of a transesterification catalyst under a reduced pressure condition, thereby making it possible to obtain an adequately highly polymerized polycarbonate resin provided with the inherent physical properties of aromatic polycarbonate resins. The following indicates an example of the specific reaction scheme of this linking and highly polymerizing reaction using an aliphatic diol compound.

On the other hand, the step for subjecting an aromatic polycarbonate prepolymer and an aliphatic diol compound to a linking and highly polymerizing reaction can also be said to be a step for producing a copolymer of the aromatic polycarbonate prepolymer and the aliphatic diol compound. In general, in the case of obtaining a copolymer by continuously copolymerizing copolymerization components with each other, all of the materials (copolymerization components or reaction components) are normally preliminarily adequately mixed over a comparatively long period of time at normal pressure with a mixer, followed by transferring to a reaction vessel and copolymerizing. In a transesterification reaction during ordinary production of aromatic polycarbonate resins in particular, although a large, horizontal stirred reaction vessel having a large reaction surface area is preferably used to enhance devolatilization effects on by-product phenol and accelerate the reaction, since the stirring capacity of the horizontal stirred reaction vessels is not large, reaction components are typically introduced into the horizontal stirred reaction vessel after having been adequately mixed in advance.
Continuous multistage polymerization methods are also known in the prior art that consist of arranging a plurality of polymerization tanks in series in production of an aromatic polycarbonate resin (Patent Documents 14 to 16).